if anyone do research on instant max stage and share if its 100% banhammer or not would be very nice
Instant max stage isn't a banhammer, since you'll just go to your max stage anyway.
There are few things to consider, tho:
1 - There's an amount of time you need to wait to do prestiges, so I'd say to you to enter the highest possible xp clan. My prestige timer is around 30 seconds to 1 minute.
2 - You won't be able to use Instat Max Stage at Tournaments, before you get to your max stage at least once, after you join the tournament.
If you try it before you reach max stage or further the game will just reload with a "error occurred" message.